Modern medical imaging methods permit physicians and researchers to more accurately diagnose, treat, and investigate a wide variety of disorders. Such imaging methods are based on various technologies including acoustic waves (ultrasound), radioactive decay (positron emission tomography), and nuclear magnetic resonance (magnetic resonance imaging). Each of these imaging techniques has its own characteristic advantages and disadvantages, but medical researchers, physicians and other practitioners continue to seek higher resolution, more reliable, less invasive, and more easily interpretable imaging systems and methods.
Magnetic resonance (MR) imaging systems generally use a static magnetic field (B0) and a radio frequency magnetic field (B1) to produce images. Unfortunately, the low signal-to-noise ratio (SNR) obtained with B0 field strengths of about 1.5 T can limit the application of this technique. Application of higher magnetic fields (for example, 3 T) can improve SNR, but these higher magnetic fields are associated with undesirable changes in off-resonance susceptibilities, magnetic field inhomogeneities, and increased specific absorption rate (SAR). Because B0 and B1 cannot be controlled with arbitrary precision, especially at high field strengths, MR signals and images can be degraded by imperfections such as non-uniformities in these magnetic fields.
One important type of MR imaging is so-called T2-weighted imaging in which image contrast is based primarily on spin-spin relaxation time constants (so-called “transverse relaxation”) referred to as T2. Conventional T2 prep sequences used to prepare a specimen for extracting a T2-weighted image consist of an initial 90° pulse to convert a substantial part of the longitudinal magnetization in the image field of view to transverse magnetization, followed by a combination of delays and RF pulses designed to refocus this transverse magnetization. Magnetization changes due to T2 relaxation accumulate during these pulses and delays. A final 90° pulse is applied to return a substantial part of the refocused magnetization to the longitudinal axis. The T2 relaxation between the application of the two 90° pulses provides the desired image contrast between sample components with different T2 relaxation rates.
Some conventional T2 preparation (T2 prep) sequences have been designed to be robust to flow as well as to inhomogeneites in both B0 and B1. Such sequences use opposing pairs of so-called Malcom-Levitt (MLEV) pulses that can compensate pulse shape imperfections in the RF magnetic field B1. Two representative sequences of such MLEV weighted composite T2 prep sequences are shown in FIGS. 1A-1B. Pulses indicated as 180x0 are composite pulses, each consisting of a 90°x180°y90°x pulse sequence. Such MLEV weighted composite pulses can compensate some imperfections in B1, with larger numbers of such pulses providing increased compensation. However, increasing the number of MLEV pulses results in an increase in specific absorption rate (SAR), thus limiting the use of large numbers of MLEV pulses, especially at high B0. Thus, MLEV pulse based T2 prep is unsatisfactory in many applications.
Combinations of T2 prep and spectrally selective fat suppression (FatSat) sequences are commonly used to enhance contrast in magnetic resonance images. In a typical T2 prep sequence, the T2 weighting is achieved by exciting the magnetization in the transverse plane with a 90 degree tip-down pulse, a train of equally-spaced composite 180 degree pulses with Malcom-Levitt (MLEV) phase cycling, and a 90° tip-up pulse. The most commonly used technique for suppression of the fat signal is based on excitation at the resonance frequency of the lipid protons. For most sequences, a narrow band RF pulse selectively excites the lipid magnetization into the transverse plane. This transverse magnetization is then dephased by a spoiling gradient to suppress the signal from fat in the acquisition sequences that follow the fat suppression sequence. However, conventional fat saturation methods are based on a chemically selective RF pulse which is typically relatively long (>10 ms at 1.5 T and >5 ms at 3 T). In some applications, such a pulse can be associated with significant increases in SAR and can require significant additional image acquisition time. In view of these and other disadvantages, improved imaging methods are needed to obtain the advantages of high field imaging with reduced sensitivity to imperfections in B0 and B1, reduced SAR, and fat saturation with reduced SAR.